Influence of surface polymer coating on ballistic impact response of multi-layered fabric composites: Experimental and numerical study
Introduction
Fabrics of various weaving based on para-aramid fibres are widely used in the manufacture of protective structures due to their high strength, stiffness and heat resistance [1]. The most famous trademarks of aramid fibres are Kevlar® (DuPont), Twaron® (Teijin Aramid), Ruslan®, Armos®, Rusar® (Russian Aramids).
Multilayer composites based on aramid fabrics for protective structures can be divided into three following groups: dry fabric panels, compliant composites with low content of thermoplastic resin, and rigid composites based on thermoset matrices like epoxy resin. The third type of composites is the least effective in terms of ballistic properties and is rarely used as an independent protective element due to the high matrix volume fraction, which significantly reduces the energy dissipation due to the friction between the yarns [1], [2], [3].
A large number of experimental and computational studies are devoted to the study of the mechanical behaviour of dry aramid fabrics during high-velocity impact [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. The authors have mentioned that friction between filaments and yarns is one of the critical factors which affect on the ballistic efficiency of aramid fabrics and the magnitude of back face bulge. Higher inter-yarn friction makes the secondary yarns more involved in absorbing the impact energy [21, [26], [27], [28]]. At the same time, the opposite effect is observed with too much pull-out resistance due to premature yarn fracture [21].
There are several ways to increase the amount of energy dissipated by the multilayer fabric target and to reduce the back face bulge:
- 1
Stitching of the fabrics [29], [30], [31], [32], [33]. The increase of the ballistic limit and the reduction of the back face bulge were observed with the right choice of stitching parameters. The disadvantages of this method include a noticeable flexibility decrease and the complexity of the selection of optimal stitching parameters.
- 2
Impregnation of fabrics with shear-thickening fluid (STF) [34], [35], [36], [37], [38], [39], [40], [41]. STF is a non-Newtonian fluid where the more shear stress rate, the higher viscosity becomes. Such behaviour is mostly observed in colloidal suspensions. When ballistic fabrics are impregnated with such colloidal solutions, the amount of energy absorbed by the composite increases not only due to the shear-thickening effect but also due to increased friction between the fabric layers and the yarns. Impact energy is distributed over a higher area in this case. The disadvantage of this method is a significant increase (sometimes more than twice) in the surface density of the composite compared to non-impregnated fabrics. To the authors' knowledge, STF has not found the application in commercial products despite a large number of publications.
- 3
Hot pressing of the fabric layers with films of thermoplastic polymers [42], [43], [44], [45], [46]. Matrix content in the composite does not exceed 20% in this case. Thermoplastic polymers like polyethylene or polypropylene have relatively low adhesion to the fibres, which allows yarns to dissipate impact energy by friction during pull-out effectively. Such materials provide excellent blunt trauma protection. At the same time, matrix presence leads to the increase of the areal density and reduces ballistic limit and flexibility.
The above-presented data demonstrate the relevance of the development of new ways to improve the effectiveness of ballistic composites with a minimum increase of surface density. Coating of aramid fabrics with thin films of polymers looks promising enough. In [47], the authors investigated the pull-out response of polymer-coated Kevlar® fabric. The pull-out test may be used as an indirect indicator of the ballistic efficiency in this case [44]. It was shown that pull-out energy normalised with areal density for coated fabrics was much higher than for the dry ones. Similar data presented in [48]. Ahmad et al. [49] demonstrated that latex coated fabric layers were effective in reducing the blunt trauma of fabric systems. In [50], it was found that the use of polyvinyl acetate for surface treatment of tissues significantly increases the coefficient of friction between threads with a slight increase in surface density.
The main goal of this study was to find a way to improve both the ballistic limit and back face bulging resistance with a slight change in the overall structure mass. The article presents the results of experimental and computational studies of the polyvinyl acetate surface coating effect on the ballistic properties of Twaron® 709 aramid fabric. The experimental part of the study included pull-out tests, high-velocity impact test of multilayer specimens without backing support, and tests on a plasticine block to assess the back face bulging. Further, the yarn-level mesoscale approach was used to analyse the mechanical behaviour of dry and coated fabrics during quasi-static and high-velocity impact tests. Obtained results confirmed a high potential of using thin polymer coatings for improvement of aramid fabric ballistic performance.
Section snippets
Materials
All tests were carried out using Twaron® 709 aramid fabric with plain weave structure and an areal density of about 200 g/m2 (Fig. 1). Mechanical properties of yarns could be found in [44]: mean values of ultimate tensile strength are 2.57 GPa/2.64 GPa on the warp and weft directions respectively, and elastic modulus is 100 GPa.
Two types of specimens were prepared for the all kind of tests:
- 1
Specimens based on the dry fabric without additional modifications (DF);
- 2
Specimens based on the fabric with
Experiments
This section describes the experiments on the yarn pulling-out, impact tests to determine the ballistic limit and plasticine indentation depth for the two types of specimens. A description and discussion of the results are also provided.
Modelling and simulations
This section contains a description of computational studies of the pull-out process and high-velocity impact loading for the two types of specimens. The mesoscale yarn-level approach was applied to create mesh models of the specimens. This approach has already been used in the number of studies to predict the mechanical behaviour of FRP subjected to ballistic impact [52], [53], [54], [55], [56]. The main goal of the calculations was to determine the possibility of using а simplified yarn-level
Conclusion
The paper considers the experimental and computational studies of the effect of polymer surface coating on ballistic impact performance of multi-layered aramid fabric composites.
It was experimentally found that by increasing the surface density of the composite by only 5–6%, it is possible to increase the ballistic limit by almost twice for specimens with the same number of fabric layers. Also, the surface coating allows reducing the indentation depth up to three times when tested on a
Author declaration
We wish to confirm that there are no known conflicts of interest associated with this publication.
CRediT author statement
Ignatova Anastasia: Conceptualization, Methodology, Investigation, Formal Analysis, Visualization, Writing - Original Draft, Writing - Review & Editing.
Kudryavtsev Oleg: Conceptualization, Methodology, Investigation, Formal Analysis, Visualization, Supervision, Funding acquisition, Writing - Original Draft, Writing - Review & Editing.
Zhikharev Mikhail: Investigation, Formal Analysis, Writing - Original Draft.
Acknowledgement
This work was financially supported by the Russian Science Foundation (project No. 18-79-00069).
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2023, Composite StructuresCitation Excerpt :After sol–gel treatment, the surface of the yarns becomes coarser and the inter-yarn friction has been improved over 20 %, and the energy absorption of 8-layer fabric increases more than 35 %. Ignatova, et al. [66] investigated the influence of surface polymer coating on ballistic impact response of multi-layered fabric composites. The results showed that the ballistic limit of the fabric specimen can be increased by almost twice through thin coatings of polyvinyl acetate.